A variety of routes for producing high purity isobutene can be employed industrially. The oldest is the sulphuric acid extraction process, but it is expensive and obsolete; it is known to be a contaminating process as waste acid is discharged. Further, the isobutene yield does not exceed 90%. The company ARCO uses tertio-butyl alcohol (TBA) dehydration, TBA being a by-product from their propylene oxide production process. The isobutane dehydrogenation process was developed during the last few years as a result of the large and increasing demand for MTBE. However, that process can only be profitable with very large production capacities.
High purity isobutene production from cracking MTBE is as suitable for small capacities as for large capacities. Further, such a route benefits from the infrastructure generated by the increasing importance of ethers in reformulated gasoline. A number of refineries throughout the world have MTBE production installations, for example. Further, there is a global exchange market for MTBE. This means that the production of high purity isobutene from MTBE can readily be carried out anywhere in the world, in and remote from refineries.
The concept of producing isobutene by decomposing an ether, more particularly MTBE, has long been known, but prior art processes have suffered from certain disadvantages.
In the process developed by SUMITOMO described, for example, in European patent application EP-A-0 068 785, the MTBE decomposition reaction is carried out in the liquid phase, in the presence of a solid acid catalyst which is an ion exchange resin. Two product streams are obtained: isobutene and methanol. As described, isobutene is obtained directly overhead from a distillation column with no other purification step. The isobutene obtained contains a certain number of impurities, beginning with a small fraction of methanol which is azeotropically distilled from dimethylether (DME), which is a volatile compound formed by condensing methanol in the presence of an acid catalyst. It is probable that the purity of the isobutene is insufficient for use in the production of polyisobutene or other copolymers. Further, there is no apparent way of avoiding the accumulation of heavy impurities such as dimers of isobutene or methyl sec-butyl ether (MSBE), which in the long run results in a fatal reduction of product purity.
In the process developed by ERDOLCHEMIE, described in United States patent U.S. Pat. No. 4,409,421, for example, isobutene is purified by eliminating residual alcohol entrained with the tertiary olefin by adsorption. This method has the disadvantage of requiring regular regeneration of the adsorbent. Further, the problem of recovering the major portion of the alcohol from the decomposition step is not solved.
More recently in U.S. Pat. No. 5,095,164, the same company has described carrying out the decomposition reaction in a distillation apparatus. The catalyst is placed in the bottom of the column at the reboiler level. That particular implementation limits the reaction temperature, which is directly imposed by the nature of the ether and the operating pressure. Further, it apparently encourages the formation of reaction by-products such as the formation of dimers of isobutene and/or dimethylether formation. In this regard, the quality and/or development of the products is not clearly explained.
In U.S. Pat. No. 4,287,379, BASF describes a scheme which integrates both ether synthesis, its separation then the ether decomposition step to produce the isobutene. However, in order to avoid certain purification steps, etherification is carried out with a C.sub.3 or C.sub.4 alcohol, which is a major disadvantage as regards the international MTBE market.
We can also cite the two flowcharts of the SNAMPROGETTI process presented in "Chemical Economy & Engineering Review", vol. 14, n 6, June 1982, including both an MTBE synthesis step and an MTBE decomposition step for the production of isobutene. Such schemes use a zone for fractionation by distillation immediately after the reactional decomposition zone. Since the treated product is rich in methanol, fractionation results in the production of two effluents each containing the alcohol: in the overhead effluent, the alcohol is entrained azeotropically, and in the bottoms effluent the majority of the alcohol obtained in the ether decomposition step is obtained. In such a scheme, alcohol recovery is thus complicated since it must be recovered both from the overhead effluent from the fractionation column and from the bottoms effluent from that column.
One method of rendering alcohol recovery easier is to carry out an alcohol extraction step immediately after the ether decomposition step. This concept is described to a greater or lesser extent in International patent application WO 91/01804 in the name of EXXON and in Rumanian patent application RO 105 954 in the name of CAROM. However, those proposed processes have a certain number of disadvantages. According to the description in application RO 105 954, the decomposition step takes place in an adiabatic reactor in the presence of steam. The presence of water in the reaction medium is deleterious to the selectivity of the decomposition reaction as isobutene is lost by reaction with water to form tertio-butyl alcohol (TBA). Further, the system also requires the provision of a supplementary decanting step. In addition, the alcohol recovery column (column C2 in the figure in that patent) is enormous due to the quantities of water used.
International patent application WO 91/01804 principally describes the possibility of regenerating the catalyst, which is preferably a clay. Thus the ether and/or alcohol extracted during the washing step is returned to the reaction section to be used as a regeneration stream. Such a batch operation, which alternates the reaction period and regeneration period in a steady frequency, results from using a catalyst which is not stable over time. The single example given concerns the decomposition of tertio-amyl-methyl-ether (TAME) to form isoamylenes, a reaction which is less demanding as regards temperature than MTBE decomposition with respect to the position of the thermodynamic equilibrium. Thus there are difficulties in operation and also difficulties in inserting it into a Generally integrated scheme where neighbouring units operate continuously (for example the etherification unit for synthesis of MTBE or TAME). It is thus necessary to store products upstream and downstream, involving additional costs and constant management.